1. Field of the Invention
The present invention relates to an optical transmitting apparatus, an optical receiving apparatus, and an optical communication system comprising them, and more specifically to an optical transmitting apparatus for transmitting an optical signal using PSK modulation, an optical receiving apparatus for receiving an optical signal from the optical transmitting apparatus, and an optical communication system comprising those apparatus.
2. Description of the Related Art
Development of a practical implementation of an optical transmitting apparatus aiming to establish a high capacity and long distance optical transmission system has been awaited in recent years. Particularly, expectations for implementation of an optical transmitting apparatus, which employs an optical modulation technique adequate for high capacity and long-distance, to an actual system are growing high. In order to meet with expectations, optical transmission systems using phase shift keying such as DPSK (Differential Phase Shift Keying) and DQPSK (Differential Quadrature Phase Shift Keying) are envisioned.
NRZ (Non-Return-to-Zero) modulation techniques and RZ (Return-to-Zero) modulation techniques, actually operated on land and under the sea, are known as actual optical modulation techniques. In an optical transmission system using such modulation techniques, technology for stabilizing operation of components in a transmitter for optical transmission signal has great importance. An example is an ABC (Automated Bias Control) circuit in the NRZ modulation for preventing transmission signal degradation caused by drift of the operating point of a LN (Lithium Niobate) modulator. (See Patent Document 1: Japanese laid-open unexamined patent publication No. 03-251815)
There is also a bias control method for an optical SSB (Single Side-Band) modulator with a plurality of optical modulation units, in which appropriate correction of direct-current bias for each optical modulation unit is performed during normal operation of the modulator. (See Patent Document 2: Japanese laid-open unexamined patent publication No. 2004-318052)
An example of an optical receiving apparatus for receiving a DQPSK signal is described in Patent Document 3 (Japanese publication of translated version No. 2004-516743). In the optical receiving apparatus in the Patent Document 3, a phase of an optical signal is shifted by π/4 in one of a pair of waveguides constituting a Mach-Zehnder interferometer.
FIG. 1 is a block diagram describing a configuration of an optical transmitting apparatus employing a conventional NRZ modulation technique with an ABC circuit for NRZ. In FIG. 1, the optical transmitting apparatus employing a conventional NRZ modulation comprises a laser diode 111, a phase modulator 221, comprising a MZ (Mach-Zehnder) modulator etc., which carries out phase modulation by inputting an NRZ data signal DATA to a modulating electrode, and an ABC circuit for NRZ 550, which, by monitoring a part of the optical output of the phase modulator 221, detects a low frequency signal superposed on the data signal DATA, applies a control signal to bias tees (not shown in figures) of the phase modulator 221, and compensates for a deviation of an operating point.
The ABC circuit in the conventional optical transmitting apparatus employing the NRZ modulation, however, only performs bias control, which compensates for the deviation of an operating point of the MZ modulator, and it does not comprise means for monitoring the amount of phase shift of a phase shift unit necessary for phase shift keying such as DQPSK, which is receiving attention for its anticipated potential. For that reason, the conventional technology shown in FIG. 1 could not be applied to phase shift keying such as DQPSK. There was a problem that the concept of total control of phase shifting and DC drift with regard to an entire optical transmitting apparatus employing phase shift keying such as DQPSK did not exist.
Additionally, in an optical receiving apparatus described in the Patent Document 3, it is required to shift the phase of an optical signal by π/4; however, no measures have been taken to prevent loss of accuracy of an optical device due to age deterioration.